Optical sensing applications are on the rise in such diverse fields as civil structures, aerospace, oil and gas, among many others. Much progress continues to be made on optical sensor technology. As sensors increase in performance and sophistication, an increasingly pressing need emerges for optical measurement equipment (e.g., sensor interrogators, including light source, detectors and wavelength references) to monitor the optical sensors with speed, accuracy and reliability over a variety of physical and environmental conditions.
Although sensor applications vary, the following sensor interrogator requirements are common for many applications. Interrogators should provide accurate and reliable, low noise wavelength measurements (preferably +/−2 to 5 pm) for many sensors (˜50-˜100's of sensors) and preferably thousands of sensors. Interrogators should be able to function with multiple channels (e.g., 4 channels and preferably be expandable to 16 channels. Interrogators should have high speed data acquisition (e.g., 250 Hz -1 KHz) with high optical power to ensure high dynamic range (DR) at high acquisition speeds and deterministic data acquisition and transfer to PC (e.g., compatibility with the TCP-IP communications protocol.) Interrogators should also have wide operating temperature range (e.g., 10-4° C., or 0-50° C.) and exhibit mechanical reliability for field applications (such as outlined in Telcordia Technologies document, GR-63-CORE). Further, the interrogators should be able to provide accurate and reliable sensor measurements even with long fiber lead-in (e.g., e.g., 100 km Round Trip).
A variety of basic interrogator system design options exist that attempt to meet the broad requirements listed above.
A system with broadband source, dispersive element, and a diode array-such systems cannot achieve the required wavelength measurement repeatability and resolution with commercially available diode arrays. Low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.
A system with broadband source and an optical spectrum analyzer (OSA) or multi-line wavelength meter-laboratory OSAs are large, slow, expensive, and do not have a wide operating temperature range. Multi-line wavelength meters acquire data at slow speeds only, and are not mechanically robust. Again low broadband source power limits the ultimate needed combination of channel count/sensor capacity and dynamic range/distance to sensors.
A system with an optical time domain reflectometer (OTDR) OTDR/TDM (time-dependent multiplexing)-the low loss budget of such systems precludes their use with the preferred larger number of sensors and/or channels, and data acquisition rates scale down with increasing sensor counts. The minimum physical grating spacing limits use of such a system in some applications.
A system with an external cavity tunable laser with power meter and wavelength meter-external cavity tunable lasers are slow, expensive, and do not have a wide operating temperature range or the required mechanical robustness. The addition of power meters and wavelength meters add to the bulk, complexity, and cost, as well as reduce reliability and speed. The polarization properties of the narrow line lasers may not be an ideal match for all sensing applications.
In contrast, an interrogator employing a swept wavelength laser source, such as that described in U.S. Pat. No. 6,449,047 offers the required optical output power for high channel and high sensor counts without compromise to either high data acquisition speed or high measurement dynamic range. Using a swept wavelength laser source, an optical sensor interrogator can collect data simultaneously on tens to hundreds of sensors across four channels at speeds from 100 Hz -1 kHz (with potential in the 2-10 kHz range) with 25 dB dynamic range for each sensor. High output power also enables a physical reach to faraway sensors, making new optical sensor applications possible. For example, given an optical fiber attenuation constant of 0.22 dB/km (e.g., for Corning SMF-2® optical fiber), the si425 optical sensing interrogator (Micron Optics, Atlanta Ga.) can accommodate fiber lead-in lengths of over 100 km, round trip. The thermal and mechanical robustness of the swept wavelength fiber laser enables a field-ready solution that meets stringent reliability requirements for thermal shock and storage, transportation and office vibration, and high relative humidity requirements, as well as the wide operating temperature range needed for many applications. Swept wavelength laser sensor interrogator designs have passed all thermal and mechanical storage and shock conditions set forth in the Telcordia GR-63, and exhibit wide operating temperature ranges of 10-40° C. and 0-50° C. High resolution detection and low noise enable the wavelength resolution and repeatability required for the most stringent optical sensing measurements. Data generated at 100 Hz -1 kHz allows for rapid and convenient averaging of sensor values to generate measurements sensitivities greater than 0.02 pm.
Thus, although several measurement methodologies can address some fraction of the listed requirements, an interrogator system employing a swept wavelength fiber laser system is uniquely capable of satisfying all of the above-listed requirements simultaneously.
Use of a swept wavelength laser as the optical source for a fiber optic sensor interrogation system has many benefits. In order to generate accurate and reliable sensor measurements, however, the swept wavelength laser of such a sensor interrogation system must be continually calibrated. Since the source is swept, the sensor wavelength as perceived in such systems is susceptible to offsets generated by the finite speed of light in optical fiber. The present invention provides a method for calibrating sensor interrogation systems, particularly those which employ swept wavelength lasers, for potentially non-linear source effects and the associate effects of the finite speed of light in transmission fiber coupled to the optical sensors.